We are planning to upgrade our CI setup to run on Xcode 27, but we want to run tests on several older iOS simulator runtimes: 26, 18, 17…
With Xcode 26.6, we had no trouble installing the needed runtimes by running:
$ xcodebuild -downloadPlatform iOS -buildVersion 18.6
$ xcodebuild -downloadPlatform iOS -buildVersion 17.5
$ xcodebuild -downloadPlatform iOS -buildVersion 16.4
When using Xcode 27.0 (27A266a), however, the command fails to work for any runtime older than iOS 26.0:
$ xcodebuild -downloadPlatform iOS -buildVersion 18.6
Finding content...
iOS 18.6 is not available for download.
At the same time, the runtimes can be installed just fine by using the "Add Platforms…" dialog from the Components pane in Xcode settings. Unfortunately, that's not an option in a headless CI machine.
The same issue also reproduces in the beta versions 27.1 (27A9269) and 27.2 (27B5019j), and regardless of macOS version (26.7 or 27.0).
Any idea how to work around the issue? I wonder if's a broken backend configuration or a regression in Xcode 27?
Posted as FB24836279.